This application claims the priority of German Patent Document 199 06 707.4, filed Feb. 18, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for determining the cylinder charge in the case of unthrottled internal-combustion engines which comprises valves with a variable valve gear.
In the case of throttled engines, the charge (air masses) of the cylinders of the engine is usually measured. For dynamic transitions, the charge is calculated by means of a model in parallel by way of a charge model and is adapted to the above-mentioned measurement. By way of a suction pipe pressure and a variable cam spread, a correction of the calculated charge can also be carried out.
However, the above-described charge model cannot be used for unthrottled engines. The reason is that there is no connection between the suction pipe pressure and the cylinder charge. However, the knowledge of the cylinder charge is required for the dynamic engine operation of engines with a variable valve gear as well as for the diagnostic and safety functions.
It is therefore an object of the present invention to indicate a method for determining the cylinder charge in the case of unthrottled internal-combustion engines which permits a determination of the charge which is as precise as possible
This object is achieved by a method for determining the cylinder charge in the case of unthrottled internal-combustion engines, which have valves with a variable valve gear, said method comprising;
determining at least one engine value,
determining a standardized air flow rate relative to a defined inlet and/or outlet spread as a function of the at least one engine value, and
determining the influence of the actual inlet spread existing in comparison to the defined inlet spread on the air flow rate and calculating a corrected air flow rate based on the determined influence.
In particular, first a standardized air flow rate is determined as a function of at least one engine value, which air flow rate is related to a fixed or known inlet and outlet spread. The engine values are, for example, the rotational speed and the height of the valve lift. In addition, the influence of the present actual inlet spread in comparison to the assumed fixed or known inlet spread is determined and a correct air flow rate is determined. In particular, by means of the above-mentioned approach, the influence of inlet (and, in the case of the advantageous embodiments) also of outlet spreads on the intake operation of the engine with a variable valve gear is described while taking into account geometrical and physical "effects". Only data of the air flow rate in the case of certain spreads must still be applied here, for example, in the form of characteristic diagrams. In a characteristic diagram, the taken-in air flow rate at fixed inlet and outlet spreads (such as 120/120) and at standard conditions (for example, 0.degree. C. and 1,013 bar) may also be filed.
In this context, it is also noted that the standardized air flow rate must not necessarily be determined from a fixed spread. It may also be read out of the characteristic diagram at an arbitrary spread. The function will then take into account the influence of the deviation of the actual spread to the desired spread. The input for the desired spread may, for example, be derived from a load-dependent characteristic diagram.
It is known that, in the case of engines with a variable valve gear, the throttling takes place by way of the inlet valve. Therefore, the air mass flowing via the inlet valve is proportional to the opened throttle surface and dependent on the differential pressure. According to an advantageous embodiment, these values are taken into account for determining the influence of the actual inlet spread. If the inlet spread therefore deviates from the standard spread, the influence of a changed inlet spread onto the air flow rate is described by the surface fraction and the differential pressure fraction. This will be illustrated more precisely in the following embodiment.
The outlet spread also has an influence on the cylinder charge. For the precise determination of the cylinder charge, the influence of the outlet spread is preferably described by the ratio of the outlet valve surface behind the upper dead center. In the following, a special embodiment of the invention will be described in detail with reference to the drawings.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.